The present invention evolved from continuing development efforts pertaining to subject matter disclosed in my co-pending Application Ser. No. 216,872, filed Dec. 15, 1980, entitled "Balun Coupled Microwave Frequency Converter" now U.S. Pat. No. 4,330,868. This co-pending Application discloses a diode mixer bridge with a port, such as the RF input port, supplied by balanced coplanar conductors.
The width of the gap between the balanced coplanar conductors affects the impedance of that port to the mixer bridge. The narrower the gap, the lower the impedance. Depending on the impedance matching constraints of a particular application, there may arise a need for a gap which is too narrow to etch within prescribed manufacturing cost considerations.
One solution to this problem is provided by the subject matter disclosed and claimed in my co-pending Application Ser. No. 243,662, filed Mar. 13, 1981, entitled "Microwave Frequency Converter with Dual Balun Port". This co-pending Application relates to improvements in the microstrip circuit layout and structure for the balanced port. The dual balun enables requisite impedance matching to certain transmission lines without resorting to extremely narrow coupling gaps between the coplanar balanced conductors.
The present invention provides another solution to the impedance matching problem between balanced conductors. The present invention provides a sufficiently narrow coupling gap to reduce the impedance between the balanced conductors. The invention also provides increased capacity between the conductors. The requisite coupling gap and enhanced capacity are accomplished in a manufacturably cost efficient manner by juxtaposing the balanced conductors on opposite sides of the substrate. The thickness of the substrate is the width of the coupling gap between the balanced conductors, and hence there is no need to etch the narrow coupling gap, which is expensive.
Balanced conductors juxtaposed on opposite sides of a substrate is itself known in the art, for example as shown in U.S. Pat. No. 3,652,941 to Neuf.
The present invention relates to improvements in microstrip circuit layout and includes linking balun structure interconnecting coplanar sections of balanced conductors to non-coplanar sections juxtaposed on opposite sides of the substrate and supplying a balanced port to the mixer bridge. This enables a cost effectively implementable coupling gap between balanced conductors supplying a balanced port for a diode mixer bridge, and yet retains the efficiencies of transmission line coupling to balanced coplanar conductors. A transmission line is juxtaposed the coplanar sections of the conductors for interacting therewith to balun couple a field balanced between the conductors.
Further linking structure connects the diode mixer bridge on one side of the substrate to two pairs of parallel coplanar grounding stubs on the other side of the substrate, each pair diverging from a respective one of the juxtaposed conductors to the ground plane. Third and fourth conductors, juxtaposed respective stubs of each pair, provide the remaining ports to the mixer.